Scanning Chemical Mechanical Polishing

ABSTRACT

A new system design for chemical mechanical polishing (CMP) is used in semiconductor microchip manufacturing. In this new design, a wafer is placed on a vacuumed stage facing up, and the wafer surface is polished by a small size polishing pad assembled to a head which sweeps back and forth across the wafer. This new design differs from a conventional CMP system which holds a wafer facing down in a head while applying the pressure to the backside of the wafer, and pushes it onto on a big platen covered with a polishing pad. This new design eliminates the complicated multiple-zone head and its expensive components, while it provides a much robust way of profile turning which is critical to the performance of microchips. This new design also minimizes the usage of consumables and therefore greatly reduces the cost of manufacturing.

INTRODUCTION

1. Technical Filed

This invention is related to a novel system realizing chemical mechanical polishing (CMP) process which is being widely used in semiconductor microchip manufacturing.

2. Background

CMP process was invented by IBM in 1980s with the purposes to meet the requirement of shrinking depth of focus in lithography patterning and to provide global flat surface to enable multiple stacking metal layer interconnections. The objective of CMP process is to totally or partially remove the unwanted pre-deposited film, and meanwhile it is critical to achieve the within-wafer remaining thickness uniformity requirement with minimum yield-killing defects like scratches, corrosions and particle residues.

Conventional commercial CMP system (FIG. 1) compromises a platen 10 which is made of hard materials like silicon carbide, and the platen 10 is covered with replaceable polishing pad 12 made of synthetic plastic. As the carrier of microchips, a wafer 14 is held in a head 16 which pushes the wafer 14 downward to the platen 10 during the polishing and suck up the wafer 14 by vacuum when the polishing is completed. The platen 10 is more than twice of the size of the wafer 14, and during the polishing both the platen 10 and the head 16 self-rotate in the same direction. Platen 10 and head 16 rotations contribute the mechanical removal of the material, while abrasives based and liquid form slurry 18 is supplied onto the pad 12 during the polishing to provide the chemical removal. An advanced commercial CMP system normally has multiple platens 10 to achieve high throughput or to remove different types of film at different platen 10, thus it occupies big and expensive clean-room space.

Head 16 with multiple-zone pressure control is being used to meet the stringent within-wafer remaining thickness uniformity requirement (FIG. 2), while the number of the zones has been increasing along with the advancing of technology nodes. However multiple-zone head 16 requires complicated pneumatic loops and controllers along with other accessories, still it is always difficult to tune the profiles at the regions between the zones. As shown in FIG. 2, a wafer 14 is held in a multiple-zone head 16 and constrained by a retainer ring 20. The membrane made airbags 22 touch the backside of the wafer 14, and the airbags 22 are divided into three ring shape zones which are named after the position: center zone 24, middle zone 26 and edge zone 28. Air 30 is supplied to each zone and higher air pressure applied onto the wafer yields higher removal rate while lower air pressure yields lower removal rate. As the examples in FIG. 2, the pressure applied in the center zone 24 is higher comparing to other zones, therefore the removal profile (of the center zone 24) 32 is faster than the profiles of other zones. While the pressure applied in the middle zone 26 is lower comparing to other zones, therefore the removal profile (of the middle zone 26) 34 is slower than the profiles of other zones. As the reference, at the edge zone 28 the removal profile (of the edge zone 28) 36 is set to be flat. Notice that when differential pressures are applied to adjacent zones, the profile at the overlapping areas 38 between the zones does not respond as the same at the center of the zones due to the limitation of physical deformation of the membrane.

The cost of CMP process is very high, not only because of the highly complicated system and the head 16, also because of many consumables which are required to be replaced at the end of the life. Those consumables are normally regarded as the slurry 18, retainer ring 20, pad 12 and conditioning disk 40.

Slurry 18 is the agent used to chemically remove the film on the surface of the wafer 14, and it comprises of small particles as the abrasives to enhance the mechanical removal mechanism along with other chemical additives and surfactants. Apparently, the minimum amount of the slurry 18 dispensed during the polishing depends on the size of the platen 10, and large part of the slurry 18 is spun off from the rotating platen 10 even before it is able to reach the surface of the wafer 14.

Synthetic plastic material based retainer ring 20 is assembled around the head 16 to prevent the wafer 14 from slipping out during the polishing, and it is grooved to allow the slurry 18 to flow through. Retainer ring 20 needs to be replaced before the grooves are worn out.

Synthetic polymer based pad 12 is placed on the top of the platen 10 to be the media touching the wafer 14 during the polishing. Grooves of various patterns are printed to the pad 12 during the fabrication, and those grooves provide the paths for slurry 18 to be transported to the entire surface of the wafer 14. The pad 12 needs to be changed before the grooves are worn out or before the removal rate drops below the acceptable level.

The pad 12 tends to generate lower removal rate after longer time polishing, and a conditioning disk 40 is used to refresh the pad 12 surface after each wafer 14 run to recover the removal rate and keep it stable within the lifetime cycle of the pad 12. Conditioning disk 40 is usually made of tiny synthetic diamonds, and it needs to be changed when the disk starts to lose its capability of sustaining the removal rate.

In summary, CMP plays a key role in semiconductor microchip manufacturing, and conventional commercial CMP systems are facing the challenges and difficulties in within-wafer remaining thickness uniformity improvement, ever complicated head 16 design and astronomical consumable costs. This revolutionary invention is targeted to provide total solutions to those issues.

SUMMARY/DISCLOSURE OF THE INVENTION

Different than polishing a head-held wafer 14 on a pad 12 covered platen 10, this invention is designed to polish a wafer 14 which is placed on a vacuumed stage 42 by a much smaller size of head 16 installed with a pad 12 of the same size. During the polishing both the stage 42/wafer 14 and the head 16/pad 12 are self-rotating, while the head 16 is also sweeping from one side of the wafer 14 to another side. The sweeping can be divided into zones which provide more flexible knobs for profile tuning After polishing, the head 16 first moves to where a brush 44 resides to clean the pad, and later to where a conditioning disk 40 resides to refresh the pad 12. A sponge 46 will move onto the wafer 14 to perform post CMP cleaning, and then the wafer 14 will be spun at high speed to be dried.

The name, “Scanning Chemical Mechanical Polishing”, is referred to the feature of a smaller size head 16 sweeping on a wafer 14 to achieve the purpose of planarization. Scanning CMP eliminates the complicated multiple zone head 16, while it provides clinical precision of profile tuning through zonal sweeping methodology. Meanwhile, Scanning CMP greatly reduces the sky-rocketing consumable cost with smaller foot-print and less consumable consumptions.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic depiction of side-view (upper) and top-view (lower) of a conventional CMP system, and is useful for comparing with the invention.

FIG. 2 is a diagrammatic sectional view of profile tuning by a multiple-zone head 16 of a conventional CMP system, and is useful for comparing with the invention.

FIG. 3 is a diagrammatic depiction of side-view (upper) and top-view (lower) diagram of a Scanning CMP system in accordance with the invention. In the top-view, it shows the edge and center positions of the head 16.

FIG. 4 is a diagrammatic sectional view of profile tuning (lower) through zonal scanning methodology (upper, in top-view and it shows the edge and center positions of the head 16) in accordance with the invention.

FIG. 5 is a diagrammatic depiction of the foot-print comparison between a conventional CMP system (upper) and a Scanning CMP system (lower) in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

The entire process of Scanning CMP consists of the steps of loading, polishing, cleaning, drying and unloading.

1. Loading

At the beginning, the wafer 14 is loaded front side (device side) up onto the stage 42, and the stage 42 will vacuum the wafer 14 tightly.

2. Polishing

The head 16 covered with a pad 12 moves onto the wafer 14 and touches the surface of the wafer 14. Down-force is applied to the head 16 through the spindle 48, while the slurry 18 is supplied to the surface of the wafer 14 at a consistent flow-rate. The head 16 and the stage 42 are self-rotating in the same direction, and meanwhile the head 16 also swings from one side of the wafer 14 to another side of the wafer 14.

The head 16 sweeping is divided into multiple zones 50 (showing eight zones in FIG. 4), and different sweeping speed and down-force can be set for each individual zone. As the examples in FIG. 4, at the most left 3 zones, slower sweeping speed or higher down-force is used, therefore the removal profile (of the left zones) 52 is faster than the profiles of other zones. In the middle 3 zones, faster sweeping speed or lower down-force is used, therefore the removal profile (of the left zones) 54 is slower than the profiles of other zones. As the reference, the removal profile of the most right 2 zones 56 is set to be flat. The boundary of an individual zone is defined as where the center of the head 16 starts and ends. Therefore the size of the overlapping area between the adjacent zones is the diameter of the head 16, and the profile of this overlapping area is impacted by the sweeping speed and down-force of both adjacent zones. Reducing the size of the head 16 will effectively minimize the overlapping area thus provides more precise corrections of the profile, while it will increase the processing time for the whole wafer 14. Therefore, the size of the head 16 needs to be decided with the consideration of balancing between within-wafer thickness uniformity performance and throughput.

An optical fiber 58 assembled in the head 16 provides real-time thickness measurement, and polishing will stop when the thickness target is reached.

3. Cleaning

When the polishing is completed, the flow of the slurry 18 is stopped, and the head 16 lifts up and moves to the cup where the brush 44 locates. The pad 12 touches the brush 44 with applied down-force while both the head 16 and the brush 44 self-rotate. This way, the slurry 18 particles that remain on the pad 12 are removed. Afterwards, the head 16 moves to the cup where the conditioning disk 40 resides. The pad 12 touches the conditioning disk 40 with applied down-force while both the head 16 and the conditioning disk 40 self-rotate. The conditioning disk 40 is stored in the water all the time to prevent it from being dried out. After the conditioning, before next wafer 14 to be loaded, the head 16 moves back to the brush 44 unit but without touching the brush 44. This way the pad 12 is stored in the water without being dried out.

After the polishing, when the head 16 moves away from the wafer 14, a sponge 46 moves in and touches the surface of the wafer 14, meanwhile chemical 60 starts to flow. The sponge 46 sweeps across the wafer 14 when the wafer 14 self-rotates at the same time. Afterwards the chemical 60 flow stops and deionization water 62 starts to flow to rinse off the remaining chemical 60. Finally, the sponge 46 lifts up and moves back to the original position where it is stored in the water to prevent it from being dried out.

4. Drying

After the cleaning, the wafer 14 is spun at high speed and with an option of hot nitrogen purging onto the surface to accelerate the drying.

5. Unloading

When the drying is completed, the stage 42 releases the vacuum, and the wafer 14 is removed from the stage 42.

The advantages of this new system are:

There is no need of multiple-zone, complicated, high cost head 16. But the capability of profile tuning is enhanced through zonal sweeping methodology.

Much less usage of the consumables. There is no retainer ring 20 at all since there is no risk of wafer 14 slipping out. The usage of slurry 18, chemical 60 and deionization water 62 is greatly reduced, since the size of the stage 42 is much smaller comparing to a platen 10 in a conventional commercial system. The size of the pad 12 is much smaller, and therefore the consumption and the cost are also low.

Small footprint. The size of the stage 42 is exactly the same as the size of the wafer 14, and the stage 42 is much smaller than a platen 10 of a conventional CMP system. Besides it eliminates the independent post cleaning and drying unit since this new system completes polishing, cleaning and drying steps in one single unit. FIG. 5 shows a comparison between this new system and a conventional CMP system. For a conventional CMP system, it usually consists of loader/unloader 64, robot/transfer 66, polishing units 68, cleaning/drying units 70 and integrated thickness measurement unit 72. This new system only consists of loader/unloader 64, robot/transfer 66, much smaller polishing units 68 and integrated thickness measurement unit 72. 

What is claimed is:
 1. A method of realizing chemical mechanical polishing process: A) A wafer 14 is sucked to a self-rotating stage 42 front-side up while a self-rotating smaller sized head 16 covered with pad 12 touching down on the wafer
 14. B) The head 16 sweeps back and forth across the surface of the wafer 14 with applied down-force to remove the surface film and to reach planarization with the assistance of slurry
 18. C) Sweeping route is divided into multiple zones 50, and profile tuning is achieved through zonal sweeping methodology which allows the settings of different sweeping speeds and down-forces for each individual zone
 50. 2. Wafer polishing using the method of claim 1, wafer cleaning and wafer drying are fulfilled in one unit.
 3. When the total removal amount is huge, it is feasible to combine the method of claim 1 with a conventional CMP system. The conventional CMP completes the bulk polishing while the method of claim 1 completes the final polishing to fine tune the final profile. 